He cheated, what should I do?

I recently found out my thoughts about my husbands strange behavior was true. He went away for job training for 4 months and during that time away, indulged in a relationship with another woman. He wined and dined her, bought her jewelry and made her feel so loved (she told me all this). I have been faithful our whole marriage and even became a stepmother to his 2 children from previous relationships. I love him, I really do. He is my first love. Is it possible that since I married so young and since he is my first love that he took me for granted. He stopped wining and dining me after the 1st year of marriage and the only jewelry he has given me is my wedding ring. I really feel that I must leave and I have no other options. This will hurt my 3 year old daughter a great deal. If I stay with him and remain stressed and unhappy due to his betrayal, I will not be my best self. Am I making the right decision? My divorce will be finalized June 15, 2010.